Structural Consequences of the Substituent Nonsymmetry on the Geometry of the Benzene Ring. Analysis of the Molecular Geometry of Diazo-Derivatives of Benzene

Abstract

Analysis of the precisely measured geometry (retrieved from the CSD) of 21 diazobenzene derivatives revealed that the CC bond in the ring cis to NNR group is significantly longer than the trans CC bond. This effect is propagated over the whole ring, resulting in the imbalance of two Kekule structure weights equal to 54.8:45.2 for the mean geometry. 6-31G* calculation of 1,3,5-tridiazabenzene in C3 symmetry shows the substantial enhancement of the effect. Approaching the NNH group to the C1C2 bond of the ring by bending the CNN bond and keeping the C2C1N angle fixed enhanced the effect considerably. By rotation around the C1N bond to the perpendicular conformation and keeping the C2C1N bond angle fixed, the π electron interactions are removed and the effect observed is due mostly to the strain. It is opposite to that of the optimized planar conformer. The conclusion is that the imbalance of Kekule structures for a ring is due to the combination of the strain effect and the π electron interaction of the diazo group with the ring.